Adaptive Traffic SignalsOverview

Kevin Fehon, P.E., PTOEPrincipal

DKS Associates

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What is “adaptive” anyway?

Signal timing is adapted to the measured traffic in real time

Traffic is measured and/or predicted, calculations are made and timing is implemented

“Adaptive” systems do not pick the best stored plan to match the measured traffic –that is “Traffic Responsive” (TRPS)

Why Bother with Adaptive Control?

Pre-stored plans accommodate variations in traffic, therefore never optimal

Time of day selection also sub-optimalPre-stored plans age quicklyTRPS difficult to optimize, most successful

on arterials, often abandoned1.5 Generation updates plans but suffers

from shortcomings of TRPS

PerformanceIndex

Performance of fixed time plans deteriorates

Performance of adaptive systems more stable

5

Time in YEARS

-15%

-10%

-5%

0%

+5%

+10%

1 2 4 53

Adaptive

Effectiveness of Fixed Timing Plans

Fixed timing plan

Performance Level

CAN ADAPTIVE SYSTEMS REALLY IMPROVE TRAFFIC OPERATIONS?

Gresham PM Peak TT(Eastbound Burnside)

405

368

424393

349 373

314

0

100

200

300

400

500

Trav

el T

ime

(sec

)

1997 Free

1998 TOD

2004 Free

2004 (1998 TOD)

2004 TOD

2007 (2004 TOD)

2007 SCATS

Legend

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Adaptive Systems Respond to Significant Changes in Demand

HOW DO THEY WORK?

Types of algorithms

Sequence basedUse a cycle length, like most coordinated signal

systemsHave a pre-defined sequence of phases (some

with flexibility)

Non sequence basedDo not use a cycle lengthDo not use a pre-defined sequence of phases

Types of systems

Complete, stand-alone, with full management system capabilities

Module within proprietary signal management system

External to proprietary signal management system

Examples of current systems

Complete, stand-aloneSCATS, SCOOT, LA ATCS

Modules for proprietary systemSynchro Green (TrafficWare/Naztec), Centracs

Adaptive, McCain QuicTrac, Intelight, OPAC (MIST)

External ACS-Lite, InSync, Rhodes

Various architectures

CentralizedAll strategic and tactical decisions at central

DistributedStrategic at central, tactical at local

Peer-to-peerNo central supervisor

How do they work?Implement one or more operational

strategy (See FHWA guidebook)Pipeline to maximize throughput

(appropriate with oversaturation)Pipeline to provide smooth flow

(appropriate with undersaturation)Equitably distribute green times

(appropriate with many turning movements)

Manage queues

Fixed or variable objective function?

A FEW EXAMPLES…

InSyncTheory

Seeks to minimize a weighted delay objective function at one critical intersection

Picks next phase (state) that will minimize short term delay function

Inserts non-coordinated phases at other intersections outside platoon bands (tunnels)

Practical limitationsMaximum wait times, overlapping tunnels, driver

expectationsEffectively runs fixed cycles at critical intersectionBest with short sections of arterial

InSync “time tunnels”

Success depends on spacing & speed

ACS-LiteTheory

Start with a standard TOD patternModify offsets based on detection of arrivals during

greenModify phase splits (based on maxouts?)Modify TOD schedule

Practical limitationsCannot adjust cycle lengthCannot accommodate traffic markedly different

from pattern basis

ACS-Lite architecture

Synchro Green

Calculation engine like SynchroLike Gen 1.5, traffic responsive with regular

recalculation of background pattern

QuicTrac

Estimate platoon speed with system detectors and calculate offsets at master

Calculate cycle length at master based on local volumes

Calculate splits at local

SCOOT

Calculation engine similar to TRANSYT/7FBased on signal timing theoryMeasures volumes entering a linkCalculates cycle length, splits and offsetsMakes regular small steps to follow changing

traffic patternsCoordinated groups fixed. Usually some VA

SCOOT platoon arrivals

SCOOT cycle by cycle changes

SCATSMeasures degree of saturation at stop line to

calculate cycle length and splitsMeasures volumes entering links to select best

direction of offsetPractical implementation of how traffic

engineers set up coordination patternsDetermines which intersections need

coordination – flexible groupingSelects appropriate objective function based on

V/C regime

Sunnyvale: Extension of SCATS

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Sunnyvale SCATS

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LA ATCS

Over 3500 intersections under controlSystem being expandedApparently operates well, but not really as

adaptive as it soundsNot well documentedBetter supported in last few years

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Other systems

Intelight adaptive (new)Surtrac (experimental – Carnegie Mellon)OPACRhodesSPOT

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What is good? What is not? SCOOT – well proven in many countries, variable in USA SCATS – well proven in many countries, generally good in

USA InSync – rapidly growing, variable results Synchro Green – steadily growing, satisfactoryACS-Lite – slow progress, several vendorsCentracs Adaptive – integrated and improved ACS-LiteQuicTrac – several successful deploymentsRHODES – development continues, practical results patchyOPAC – variable results, not growingLA ATCS – apparently good, little published data, expanding

beyond City of LA.

Is an adaptive system right for you

Do you have a corridor or closed networkDoes demand exceed the theoretical and

practical capacity of your corridor

Should I consider adaptive control?

What do the opponents say?I know exactly what will happen on my street on

Monday morning.The transitions cause too much disruption and are

counter productiveThe detection costs too much and is not reliableThe communication costs too much or is unreliableI don’t believe the claimsI have too much investment in my current system

How should I get an adaptive system?If any Federal funds involved, must

provide a systems engineering analysis, commensurate with the scale of the project

Selection must be competitive, but recommend AGAINST low-bid process

You are buying technology, with every product different, not buying pavement complying with a physical specification

Procurement optionsBest Value

Low Bid with SE Support

“Consumer Reports"

Would you do it again?

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Source: Adaptive Traffic Control Systems in the United States, Matt Sellinger, HDR Engineering, Inc.